The epoxy materials have a high crosslinking density, which provides them with a high glass transition temperature Tg, which confers excellent thermomechanical properties on the material. The higher the crosslinking density, the higher the Tg of the material and consequently the better the thermomechanical properties: the higher the operating temperature limit of the material. Nevertheless, the impact strength properties of epoxy materials are insufficient for numerous applications. Numerous solutions have been developed to attempt to respond to this problem. At the same time, while all epoxy materials are difficult to strengthen with regard to impacts, epoxy materials with high Tg values are the most difficult. Numerous studi s have been devoted to the impact strengthening of these epoxy materials with high Tg values and these studies conclude that the addition of rubber to an epoxy material with a high Tg value does not have a strengthening effect. Mention may be made, as examples of such materials, of BADGE/DDS systems (Tg=220° C.), in which DDS denotes diaminodiphenyl sulphone, or BADGE/MCDEA systems (Tg=180° C.), in which MCDEA denotes 4,4′-methylenebis(3-chloro-2,6-diethylaniline). In the preceding materials, BADGE denotes bisphenol A diglycidyl ether.
The addition of Reactive rubbers (ATBN, CTBN) has been disclosed.    These abbreviations mean:    CTBN: Carboxyl-terminated random copolymer of butadiene and acrylonitrile,    ATBN: Amino-terminated random copolymer of butadiene and acrylonitrile.
These products are oligomers based on butadiene and on acrylonitrile which are terminated either by carboxyl functional groups or by amine functional groups. Butadiene has a very low Tg, which is favourable for producing good strengthening with regard to impacts, but it is immiscible with epoxy resins. A certain percentage of acrylonitrile is copolymerized with the butadiene in order for the product formed to be initially miscible with the epoxy resin and thus to be able to be easily incorporated in the latter. P. Lovell (Macromol. Symp. 92, pages 71-81, 1995) and A. Mazouz et al., (Polymer Material Science Engineering, 70, p. 13, 1994) say that, on conclusion of the crosslinking reaction, a portion of the functional oligomer forms elastomer particles and a not insignificant portion remains incorporated in the matrix. This is reflected by a fall in the Tg of the material obtained with respect to the pure epoxy network, which is undesirable for applications requiring good thermomechanical properties. The elastomer domains formed have a large size conventionally of between 0.5 microns and 5 microns. The strengthening obtained is not satisfactory.
For all these reasons, other solutions for the impact strengthening of epoxy networks have been sought. Mention may be made, for example, of P. Lovell (Macromol. Symp. 92, pages 71-81, 1995), who establishes that strengthening with preformed core-shell particles leads to better results.
As regards strengthening with preformed core-shell particles: these are preformed particles with an elastomer core, with a glass transition temperature of less than −20° C., and a rigid shell, with a glass transition temperature of greater than 50° C., which may or may not carry reactive functional groups. A reactive functional group is defined as a chemical group capable of reacting with the oxirane functional groups of epoxy molecules or with the chemical groups of the hardener. Mention may be made, as non-limiting examples of reactive functional groups, of: oxirane functional groups, amine functional groups or carboxyl functional groups. These particles of well defined size are added to the reactants (epoxy and hardener). After reaction, the material formed is characterized by a dispersion of these particles within the thermoset matrix. The elastomer particles in the material obtained have the same size as at the start, before the reaction. This result is well known; mention may be made, as examples of the prior art describing it, of, for example, the article by Maazouz et al., Polymer Bulletin 33, pages 67-74, 1994, and by Sue et al., Rubber-Toughened Plastics, 1993, pages 259-291 (cf. page 261).
These preformed particles are obtained by a two-stage emulsion synthesis; the elastomer core is synthesized during the first stage and the shell is grafted onto the core during the second stage. This synthetic process results in particles with a core size varying between 30 nanometres and 2 microns (Sue et al., Rubber-Toughened Plastics, 1993, pages 259-291 (cf. page 261)). Numerous studies have been devoted to determining the size of the elastomer core of the particle for producing optimum impact strengthening. These studies show that, with preformed particles, satisfactory strengthening can only be obtained for particle sizes of greater than 120 nanometres.
Given the size of the elastomer domains in the material obtained, the latter is not transparent. This opaqueness is an impediment in some applications. This is the case, for example, with applications of thermoset materials in composites where the manufacturer wishes to be able to visually observe the quality of his structure (thermoset material+fibres or thermoset material+fillers). Mention may also be made of the example of electronic applications of epoxy materials; the opaqueness of the material is harmful as it is an impediment to the user.
The prior art has also described the addition of a PEO-PEE Diblock:    Hillmyer et al. (M. A. Hillmyer, P. M. Lipic, D. A. Hajduk, K. Almdal, F. S. Bates, Journal of the American Chemical Society, 1997, 119, 2749-2750) have carried out studies on mixtures of a thermosetting epoxy/phthalic anhydride system and of an A-B diblock, where A is poly(ethylene oxide) and B is poly(ethylethylene), PEO-PEE. These authors have shown that the material obtained is characterized by a very specific morphology. It is composed of a thermoset matrix in which are evenly distributed PEE cylinders all having the same diameter of 5 to 10 nanometres, the cylinders themselves being surrounded by a shell (or by a sheath) of PEO with a thickness of a few nanometres. The authors found that the materials obtained were transparent but they did not study their properties nor allude to the properties which they might exhibit.
The addition of a PEO-PEP diblock to a BADGE-MDA system has also been described (Lipic P M, Bates F S and Hillmyer M A, Journal of the American Chemical Society, 1998, 120, 8963-8970). MDA denotes methylenediamine. The studies and the results are equivalent to those in the preceding paragraph.
The addition of a Polysiloxane-Polycaprolactone block copolymer has also been described: PCL-b-PDMS-b-PCL and (PCL)2-b-PDMS-b-(PCL)2. Könczol et al. (Journal of Applied Polymer Science, vol. 54, pages 815-826, 1994) have studied blends between an epoxy/anhydride system and a PCL-b-PDMS-b-PCL or (PCL)2-b-PDMS-b-(PCL)2 multiblock copolymer, where PCL denotes polycaprolactone and PDMS polydimethylsiloxane. The authors show that the material obtained is transparent and that the addition of 5% to 15% of copolymer makes possible a significant improvement in the impact strength of the epoxy material.
The prior art also refers to the use of block copolymers in compatibilizing thermoplastic/thermoset systems. Thus, Girard-Reydet et al., Polymer, 1999, No. 40, page 1677, have studied thermoplastic/thermoset blends where the thermoplastic is either PPE (polyphenylene ether) or PEI (polyetherimide) and the thermoset system is the BADGE/MCDEA pair. These blends are brittle. The authors have found that the use of a maleized copolymer comprising SEBS blocks, modified beforehand by reaction with a monoamine or a diamine (such as MCDEA), made it possible to improve the impact strength of the thermoplastic/thermoset blend.
The addition of block copolymers having at least one block predominantly composed of methyl methacrylate units has not been disclosed in the prior art. It has now been found that the addition of block copolymers having at least one block predominantly composed of methyl methacrylate units to thermoset materials results in thermoset materials with improved impact resistance. Furthermore, these materials remain transparent and the Tg is maintained or is not lowered by more than 12° C. It is possible, in addition to the block copolymer predominantly composed of methyl methacrylate units, to add other block copolymers or impact modifiers, such as core-shells or functionalized elastomers. Depending on the nature of these modifiers added in addition, the material may not remain transparent but the impact strength is very greatly improved.